Dealing with salinity in Wheatbelt Valleys - Department of Water

Finally, large 'lake fields' or boinkas occur in the
Eastern and South-Eastern Wheatbelt, where
primary salinity or 'natural' groundwater discharge
features are common. Lake chains such as Lake
Grace and Lake King and 'lakes' areas north-east of
Esperance are examples of these systems.
Playa formation remains subject to speculation;
however there are common elements in most
hypotheses. Playas occur in arid environments
where groundwaters are shallow. They appear to be
initiated at a point of impounded surface flow (such
as could be due to differential compacting of
sediments or geological barriers). They experience
deflation following denudation (due to phases of
aridity and salinisation), there remains a supply of
sediment, regular inundation (prevents vegetation
colonising the areas) and there is sufficient erosive
capacity for its continued removal (particularly wind).
Killigrew & Gilkes (1974) reviewed the
geomorphology of 600 Wheatbelt and Goldfields
playas, most with an area of 0.05 km 2 . They found
that the orientation of the lakes varied from north to
south within the Wheatbelt, that sediment moved in
littoral currents and that playa features display
equilibrium with contemporary fluvio-aeolian
processes. In other words, given initiation, flooding
frequencies shaped and orientated playas. However,
Bowler (1976) and co-workers observed that the
current size of lakes is much less than that which
occurred previously. In Western Australia, most
playas have characteristic lunette fields and strand
lines of lakes, indicating that current lakes are
perhaps only 50% the size they were at the onset of
the last dune building phase (< 20,000 BP), e.g. Lake
Toolibin and Taarblin (George & Dogramaci, 2000).
Groundwater discharge from playas takes place by
passive mechanisms, e.g. evaporation. However in
some lakes, groundwater flow can be observed as it
issues from the aquifer around the circumference of
the lake, especially from areas of highest adjacent
gradient. Macumber (1991) identified that lakes in
southern Australia, particularly larger systems,
developed seepage faces that resulted from
groundwaters being driven up salinity or 'density
ramps' beneath the lake. He showed that the
majority of evaporation occurred towards the floor
of the lake and 'reflux' (density driven downward flux
of brines to deep sediments) of brines occurred to
the aquifer below.
Unfortunately, few playas have been studied in the
Wheatbelt; however in the Goldfields several playas
have been studied in the process of extracting
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George and Coleman
minerals. Most of the information is the subject of
'commercial in confidence' reports but several
publications have been made on palaeorivers and
their potential impact on the playas. Examples of
relevant articles are Turner et al. (1992), McArthur
et al. (1989) Timms (1992) and the thesis by Johnson
(2000). Unpublished work at Lake Carey, south of
Laverton suggests that Lake Carey does not overlay
palaeochannels, which are to the east of the main
playa. An order of magnitude mass balance suggests
that the hydrological interaction between the two is
small and that the key components in the size and
form of the playa are more to do with surface runoff
and interactions with the superficial groundwater.
The playa and its sediments (~100 metres to the
basement) act as a sink or reservoir for salt. The salt
load in the sediment of the playa was equivalent to
40,000 years of salt load from rainfall. This is a
similar figure to what was calculated for wheatbelt
environments. The goldfield playas tend to have a
much higher island to surface area ratio than the
wheatbelt playas with many of the islands formed by
arid-phase sedimentary processes and maintained by
current fluvial and aeolian events.
Playas offer an opportunity for the storage of
episodic flood runoff and drainage waters. The
impact on the groundwater hydrology of lakes and
the surrounding landscape (salinity risk) has not been
adequately studied. However comparative studies of
small farm dams have shown that leakage (even if
small) causes a groundwater mound to develop,
reducing the hydraulic gradient and increasing
groundwater discharge nearby. Full playas may also
affect watertables in this way. However the impact
will be lessened if the hydroperiod (inundation time
and frequency) is short, the salinity is relatively low,
waterlevels remain below the 'active discharge zone'
surrounding the lake and surface water management
mechanisms are in place (overflow, bypass and
similar structures).
Ecology of playas
Until relatively recently, salt lakes and pans, salt
marshes and coastal samphire flats have been
perceived as undesirable places, full of mosquitoes,
unsuitable for farming and only useful for rubbish or
fill and associated housing sub-divisions. Encroaching
salinisation is impacting natural remnants (George et
al. 1995) and the degradation to naturally saline but
highly variable ecosystems has also increased. With a
shrinking conservation estate, these 'saline wetlands'
are now beginning to be seen as more important
vestiges of wheatbelt ecology, to be protected and
preserved.